A. Field of the Invention
This application relates to liquid crystal devices, particularly devices employing ferroelectric liquid crystals.
B. U.S. Pat. No. 4,367,924
In U.S. Pat. No. 4,367,924 (hereinafter "said patent"), the contents of which are incorporated herein by reference, a liquid crystal electro-optic device is described employing a chiral smectic C or H ferroelectric liquid crystal. In that device the liquid crystal is disposed between parallel plates with the planar smectic layers normal to the plates (see said patent, FIG. 2). These smectics are characterized by an average molecular long axis direction, indicated by the molecular director, n, which is constrained, in equilibrium, to make some temperature dependent angle, .psi..sub.o, with the normal to the layers, but which is free to take up any value of the angle .phi. which gives the orientation of n about the layer normal. Typically .psi..sub.o, which is a property of the bulk smectic, is in the range from 0.degree. to about 45.degree.. The ferroelectric polarization, P, reorients with n, always remaining locally normal to n and lying parallel to the plane of the layers, as shown in FIGS. 1 and 2 of said patent.
In the device described in said patent, the plates were treated so that the molecules near the plates would adopt an orientation having the average molecular long axis direction parallel to the plane of the plates but free to adopt any orientation within that plane. That is, the molecular director, n, is constrained at the surface to lie in the surface plane. This condition, when combined with the additional constraint that the director make the angle .psi..sub.o with the normal to the layers (see said patent, FIG. 2), leads to a geometry in which, if the plates are sufficiently close together, the intrinsic helical configuration of n which is present in the bulk will be suppressed, leaving two surface stabilized states of the molecular orientation configuration, each having the ferroelectric polarization normal to the plates but in opposite directions (see said patent, FIG. 2). Devices such as this, which employ surface interactions to stably unwind the spontaneous ferroelectric helix, will be referrer to as Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) devices.
The device of said patent exhibits several novel features which distinguish it from other liquid crystal devices:
(1) Optic axis rotation about the sample normal--A ferroelectric smectic in this geometry, behaves optically as a biaxial slab with the optic axes nearly along the director orientation. The biaxiality is generally weak, so the behavior is essentially uniaxial with the uniaxis along the director. The effect of switching is to rotate the uniaxis about the normal to the surface through an angle of twice the tilt angle .psi..sub.o. This is the only liquid crystal parallel-plate geometry allowing a rotation of the uniaxis of a homogenous sample about the surface normal.
(2) Strong-weak boundary conditions--Another unique feature to be noted is the nature of the required boundary condition. In order to obtain bistability, boundary conditions which constrain the molecules to be parallel to the plates but allow several or continuous orientations about the normal to the plates are required. The device of said patent is the first liquid crystal electro-optic structure to employ such a combination of strong and weak boundary conditions. A consequence of this feature and an essential property of the structure is that the director at the surfaces is switched between stable surface orientation states as an intrinsic part of the overall switching process. The SSFLC is the first liquid crystal electro-optic structure wherein switching between stable surface states has been demonstrated and the first case in which ferroelectric liquid crystal domains have been made to appear.
(3) A significantly higher switching speed--As a result of having the helix unwound, it is the first ferroelectric liquid crystal device to achieve the minimum, intrinsic response time for molecular reorientation to a changing electric field, since, with the helix unwound, bulk reorientation can occur without the motion of topological defects in the orientation field.